1. Field of the Invention
The present invention relates to a plasma addressed electro-optical display having a flat panel structure formed of a liquid crystal cell and a plasma cell affixed together with a dielectric sheet interposed in between. More particularly, it relates to a technology to prevent crosstalk from occurring in a plasma addressed electro-optical display.
2. Description of the Related Art
A plasma addressed electro-optical display panel is disclosed for example in Japanese Patent Laid-open No. Hei 1-217396 and FIG. 8 shows its structure. The plasma addressed electro-optical display panel has a flat structure formed of a liquid crystal cell 101 and a plasma cell 102 affixed together with a dielectric sheet 107 made of a thin glass plate or the like interposed in between. The plasma cell 102 is formed with a glass substrate 104 on the lower side and hermetically joined to the dielectric sheet 107 with a predetermined space in between. An ionized gas is contained in the hermetically sealed space. The glass plate 104 is provided on the lower side on the inner surface thereof, with a plurality of pairs of discharge electrodes 108 and 109 disposed parallel to each other. Each pair of the discharge electrodes 108 and 109 function as the anode and the cathode for ionizing the hermetically sealed-in gas to generate plasma discharge and, thus, they form a discharge channel. On the other hand, the liquid crystal cell 101 is provided with a liquid crystal layer 106 sandwiched between the dielectric sheet 107 and a glass substrate 103 on the upper side. The upper substrate 103 is provided, on the inner surface thereof, with signal electrodes 105 in a striped array. The signal electrodes 105 cross the above described discharge channels at right angles. The signal electrodes 105 function as column driving units and the discharge channels function as row scanning units and, at the intersections of them, there are defined pixels in a matrix array and an image is formed by the pixels in the matrix array.
The operating principle of the plasma addressed electro-optical display panel shown in FIG. 8 will be briefly described with reference to FIG. 9. In this panel, while the discharge channels in which plasma discharge takes place are selectively scanned in a line sequential manner over the screen, signal voltages are applied to the signal electrodes 105 on the liquid crystal cell side in synchronism with the selective scanning, and, thereby, a desired picture is displayed. To achieve this, a scanning circuit 201 is connected to the plasma cell side and a signal circuit 202 is connected to the liquid crystal cell side. When plasma discharge occurs in the discharge channel formed of an anode A and a cathode K, the potential therein is maintained virtually at the anode potential. When, in this state, a signal voltage is applied to the signal electrode 105, a signal voltage is written into the liquid crystal layer 106 of each pixel through the dielectric sheet 107. When the plasma discharge is ended, the discharge channel is left at a floating potential and the signal voltage written therein is held by each pixel. A so-called sampling is thus performed and, while the discharge channel functions as a sampling switch, the liquid crystal layer 106 functions as a sampling capacitor. The transmittance factor of the liquid crystal varies with the signal voltage used for the sampling and, thus, activating and extinguishing the plasma addressed electro-optical display panel is carried out by pixels as the units.
FIG. 10 is a drawing schematically showing only a portion of the signal electrodes 105 formed on the inner surface of the substrate 103 on the side of the liquid crystal cell 101. Although a color filter is placed over or under the signal electrodes 105 in constructing a color plasma addressed electro-optical display panel, it is not shown in the drawing. However, for the sake of explanation, colors assigned to the signal electrodes 105 are denoted by "Red", "Green", and "Blue". Now, if a cyan color is to be displayed in a normally white mode, the display of the color cyan can be achieved by applying a signal voltage, for example, of +70V to the signal electrode which is identified as Red, as shown in FIG. 10, and a signal for displaying the black image is applied to the Blue and Green electrodes. At this time, the adjoining Blue and Green signal electrodes receive horizontal electric fields as indicated by the arrows in the drawing. The liquid crystal existing between the signal electrode Red and the signal electrode Blue and between the signal electrode Red and the signal electrode Green receives the electric fields virtually in the horizontal direction and changes its molecular alignment. This phenomenon is called crosstalk. Since the liquid crystal at these portions is not necessary for the originally intended display of the color cyan, it is normally shut out from the visual field by placing black masks at the corresponding portions of the color filter. However, since, in the case of the plasma addressed electro-optical display panel, the electric field is applied to the liquid crystal through the dielectric sheet 107 (refer to FIG. 8 and FIG. 9), the signal voltage applied to each signal electrode 105 is set high. Therefore, the amount of crosstalk becomes much greater than in a normal active matrix type liquid crystal display panel or the like. Accordingly, the horizontal electric fields not only affect the gap portions between the adjoining signal electrodes but also affect the effective edge portions of the signal electrodes beyond the gap portions. If it is attempted to cover such effects with black masks, a considerable quantity of light is shielded thereby and it becomes impossible to provide a sufficient quantity of light for the display. Since such crosstalk is caused by leakage of the signal voltage, crosstalk appears most conspicuously at the time of displaying of the gray image which is performed by driving the liquid crystal at the portion of the applied voltage/transmittance characteristic where the slope is steep, i.e., in the range where the liquid crystal is more sensitive to the applied voltage (the voltage at this time is called a half-tone voltage).
When a horizontal band in a gray color is to be displayed on a screen with a background of a cyan color as shown in FIG. 11, the scanning is started in turn from the top of the screen and the color cyan is written in first. At this time, the liquid crystal existing between the signal electrodes receives the horizontal electric field at a considerably high level. Then, in the period when the gray band is written, half tone voltages at equal levels are applied to the electrodes of all the three colors of Blue, Red, and Green, and therefore the electric field is not applied in the horizontal direction. Then, with the progression of the scanning, the color cyan is written in as the remaining portion of the background and, thus, one frame is projected. Since the liquid crystal is actuated by the effective value of the applied voltage, when such an image is displayed, the liquid crystal existing between the adjoining signal electrodes at the edge portions of the differently colored regions suffers changes in the molecular alignment because most portions of it have been driven by the color cyan. Consequently, in the edge portion of the gray band, there is made a display in a color different from that (gray) originally intended. In the ordinary plasma addressed electro-optical display panel, there are arranged a great number of signal electrodes at narrow intervals. In order to improve the transmittance of the incident light, it is preferred that the gap between the signal electrodes be as narrow as possible. However, the phenomenon of leakage of voltage between adjoining signal electrodes, called crosstalk, appears more conspicuously as the gap is narrowed. Due to such a phenomenon, not only is color reproducibility deteriorated, but also stripes of different color appear at the portions above and below the window frame when a window is displayed on a monitor screen of a personal computer or the like. In the case of the plasma addressed electro-optical display panel, on account of its structure, it must be supplied with ten or more times as high a liquid crystal driving voltage as in a liquid crystal display panel of an active matrix type using thin-film transistors or the like. Hence, the amount of crosstalk becomes much greater than in the liquid crystal display panel of the active matrix type, and this causes deterioration in the quality of display and constitutes a problem that is to be solved.
A further description of crosstalk will be given with reference to FIG. 12. To make the description easier to understand, the parts of the plasma addressed electro-optical display panel shown in FIG. 12 are each denoted by reference numerals corresponding to those used for the plasma addressed electro-optical display panel shown in FIG. 8. The voltage applied to the signal electrode 105 is determined according to the picture data of each pixel on the basis of the applied voltage/transmittance characteristic of the liquid crystal layer 106. For example, in the case where the plasma addressed electro-optical display panel is driven in a normally-white mode, when a monochromatic green color is to be displayed all over the screen, a signal voltage of about 20V is applied to the signal electrodes to which Green is assigned by the color filter 150. To the adjoining signal electrodes to which Red and Blue are assigned, a signal voltage of about 80V is applied. Since the signal voltages are applied also at other timing than that when a discharge channel 130 is selected, leakages of the electric field in the horizontal direction as shown by the arrows are generated. As a result of the leakages of the electric field, the effective values of the signal voltages applied to the pixels affect the pixels such that their transmittance is lowered. Hence the transmittance of the green color to be displayed is lowered.
As another example, a case where a green-colored window is displayed with a white background on a screen 170 as shown in FIG. 13 will be mentioned. The same effects as described above are produced at the portions located above and below the window in the white background and the luminance at these portions is lowered. Especially in this case, while the electrodes to which Green is assigned receive effects from both sides, the signal electrodes to which Red and Blue are assigned are at the same voltage and, hence, they are affected only by the signal electrodes on one side thereof to which Green is assigned. Consequently, the portions located above and below the window that are to be displayed white come to have transmittance levels which are differently lowered by colors and, hence, lowered luminance with colors appears there.